Rows
Nine cells across. Every digit 1–9 appears once, with no repeats.
Sudoku Rules
A concise, canonical reference to the rules of Sudoku.
The rule of Sudoku
Sudoku has a single rule, applied three ways. Each row, each column and each 3×3 box must contain the digits 1 through 9 exactly once. No digit may repeat within any row, column or box.
The three constraints
Columns
Nine cells down. Every digit 1–9 appears once, with no repeats.
Boxes
Each 3×3 block contains every digit 1–9 once, with no repeats.
What makes a valid puzzle
- Unique solution. A proper Sudoku has exactly one solution. A puzzle with two or more answers is considered invalid.
- Logically solvable. The single solution can be reached by deduction; well-made puzzles never require guessing.
- Minimum clues. A standard 9×9 Sudoku with a unique solution needs at least 17 given numbers — proven in 2012.
Quick cheat sheet
- Fill every empty cell with a digit 1–9.
- No repeats in any row.
- No repeats in any column.
- No repeats in any 3×3 box.
- Use logic, never guesswork — there is always one provable answer.
Rules clear? Put them into practice on an easy grid, or read the step-by-step beginner's guide for a worked example.
Frequently asked questions
Can a Sudoku have more than one solution?
A proper Sudoku has exactly one. Multiple solutions mean the puzzle is invalid.
What is the minimum number of clues?
A unique 9×9 Sudoku needs at least 17 givens. This was proven by exhaustive computer search in 2012.
Do the same rules apply to 4×4 and 16×16 Sudoku?
Yes — fill each row, column and box with the full set of symbols once. Only the grid size and symbol set change.